In order to open and close the chuck jaws of a powered chuck, a rotating cylinder is mounted to the spindle drive pulley, and this cylinder rotates with the spindle. In a conventional power chucking system hydraulic power is supplied to drive the cylinder axially, and this axial movement of the cylinder is transmitted to the chuck by means of a rod or tube called a “drawbar” or “drawtube”. Thus, the grip of the jaws on the workpiece is determined by the hydraulic pressure applied to the cylinder.
This conventional power chucking system suffers from a number of disadvantages, including those arising from the fact that a separate power source is required to drive the rotating cylinder (chuck actuator), namely increased numbers of parts and expense, and increased power consumption due mainly to shearing of oil as the cylinder rotates. Also, the pressure of the rotating cylinder is manually adjusted and can be adjusted to pressures well in excess of the chuck capabilities, which can result in mechanical failure. Other disadvantages include grip hysteresis resulting from stopping a chuck rotating at speed and then starting it again, which can lead to an increase in grip and hence distortion of the workpiece, the safety requirement for additional sensors to detect when the cylinder is near the end of the stroke, and possible loss of grip in the event of loss by pressure to the cylinder.
It is an aim of the present invention to overcome at least one problem associated with the prior art whether referred to herein or otherwise.